The present invention relates to a planar structure HgCdTe photovoltaic detector and to its production process. This detector, whose semiconductor material HgCdTe has a small forbidden band width is particularly suitable for detection of infrared radiation.
The semiconductor compound is in fact a solid solution of mercury telluride (HgTe) and cadmium telluride (CdTe) of formula Hg.sub.1-x Cd.sub.x Te. The parameter x between 0 and 1, commonly called the composition, fixes the cadmium telluride: mercury telluride ratio. It also fixes the forbidden band width Eg of the compound and this in turn fixes the spectral detection range of the radiation. A forbidden band width Eg makes it possible to detect photons of wavelength below the cut-off wavelenth .mu..sub.c of the detector with .lambda..sub.c =1.24/Eg (.lambda..sub.c is expressed in .mu.m and Eg in eV).
Composition x can be continuously adjusted, thus making it possible to cover a vast spectral range from .lambda..sub.c =0.8 .mu.m (x=1) to very great wavelengths (Eg=0 for x.apprxeq.0.17), detections up to a wavelength of 30 .mu.m having been demonstrated.
Usually greatest interest is attached to spectral ranges adapted to atmospheric transmission windows, e.g. windows of wavelengths 3 to 5 .mu.m and 8 to 12 .mu.m. These transmission windows are in fact well adapted to thermal imaging, because at temperature close to ambient temperature (290.degree. K.) objects have an inherent emission maximum at a wavelength of about 10 .mu.m. Compositions x close to 0.30 and 0.20 respectively correspond to the spectral ranges of 3 to 5 and 8 to 12 .mu.m.
The compound HgCdTe has been universally adopted for producing detectors equipping high performance infrared systems. These detectors are either of the photoconductive type, or of the photovoltaic type. The present invention relates to devices of the second type.
HgCdTe would also appear to be the most promising for producing mosaics of integrated electronic raading detectors for equipping future infrared systems. Thus, it makes it possible to produce high quality photovoltaic detectors permitting an optimum coupling to the reading circuit with a very low dissipation.
Brief reference will now be made to the various performance criteria to be respected by a photovoltaic detector. The most important parameters, which appear in the attached FIG. 1, showing the characteristic I=f(V) of such a detector are:
a short-circuit current or photocurrent I.sub.c, which is a function of the incident photon flux,
a dark current I.sub.o corresponding to the reverse bias of the detector, which must be small compared with the photocurrent I.sub.c,
a zero bias dynamic resistance R.sub.o, which must be high,
a shunt resistance R.sub.s corresponding to a dynamic resistance when the detector is reverse biased and which must be high for an optimum coupling to the electronic reading circuit and for minimizing the noise of the reverse biased detector and
a high avalanche voltage V.sub.A.
All these performances naturally impose an upper limit to the use temperature, which becomes smaller as the forbidden band length Eg decreases and the cut-off wavelength .lambda..sub.c is high. In particular, R.sub.o varies as exp(-Eg/kT) or exp(-Eg/2 kT) in accordance with the use temperature range, k being the Boltzmann constant and T the temperature in .degree.Kelvin.
However, towards low temperatures, there is a ceiling to performances as a result of the appearance of currents which are independent of the temperature and which are often linked with leakage current phenomena on the surface of the detector. The origin of these leakage currents can be an interband tunnel effect, which increases very rapidly with the cut-off wavelength. High performances may be sought towards low temperature, particularly under ambient conditions with a limited photon flux, e.g. in infrared astronomy.
Initially, using planar technology, the photovoltaic detectors were produced in the form of a single PN junction in the HgCdTe semiconductor material. The attached FIG. 2 shows a planar structure HgCdTe detector. This detector comprises on a monocrystalline CdTe substrate 1 a monocrystalline layer 3 of Hg.sub.1-x Cd.sub.x Te of type P with 0&lt;x&lt;1 having a diffused type N region 5 constituting the active zone of the detector. The semiconductor layer 3 is surmounted by an insulating coating 7, e.g. of ZnS having a window 9 permitting electric contacting on the type N region 5. Contacting is ensured by a contact element 11, e.g. of chromium-gold. Such a detector is more particularly described in "Semiconductors and Semimetals" vol. 18, Mercury Cadmium Telluride Chapter 6, pp.201 . . . Academic Press, 1981.
Unfortunately, these simple planar structure detectors had excessive dark currents I.sub.o and consequently inadequate dynamic resistances R.sub.o, which limited their spectral range and consequently their use.
In order to improve the performances of HgCdTe photovoltaic detectors, consideration was given to producing them in the form of heterojunctions with the PN junction with a small forbidden band width and a PN junction with a greater forbidden band width. In the article by P. MIGLIORATO and A. M. WHITE, which appeared in Solid State Electronics, vol. 26, no.1,pp.65 to 69, 1983 and entitled "Common anion heterojunctions: CdTe-CdHgTe", a description is given of N cDTe/P Hg.sub.1-x Cd.sub.x Te heterojunctions with 0&lt;x&lt;1 and more generally heterojunctions N Hg.sub.1-x.sbsb.1 Cd.sub.x.sbsb.1 Te/P Hg.sub.1-x.sbsb.2 Cd.sub.x.sbsb.2 Te respectively with the forbidden band width Eg.sub.1 and Eg.sub.2, Eg.sub.1 being smaller than Eg.sub.2 and in which x.sub.1 is smaller than x.sub.2.
This type of heterojunction can be schematically represented as in FIG. 3. It comprises a monocrystalline type P Hg.sub.1-x.sbsb.1 Cd.sub.x.sbsb.1 Te monocrystalline layer 13 surmounted by another type p Hg.sub.1-x.sbsb.2 Cd.sub.x.sbsb.2 Te monocrystalline layer 15 having a type N implanted or diffused region 17.
In such a heterostructure, the connection of the energy bands follows the common anion law, which essentially fixes the valence band. This valence band has no gap at the heterojunction, the gap Eg.sub.2 -Eg.sub.1 being transferred to the conduction band. Moreover, the heterojunction is abrupt, i.e. one passes abruptly from composition x.sub.2 to composition X.sub.1.
In view of the discontinuity of the conduction band, photons of a great wavelength cannot be detected. Thus, energy photons between Eg.sub.2 and Eg.sub.1 can only be absorbed in the material of composition x.sub.1. Electrons produced during an irradiation of the structure cannot traverse the heterojunction because they encounter a potential barrier.
MIGLIORATO and WHITE proposed surmounting this problem by a gradual heterojunction making it possible to eliminate the potential barrier opposing the passage of the heterojunction by electrons. The potential barrier disappears for a gradual N doping which is sufficiently high in the part of composition x.sub.2 and sufficiently gradual variations at the heterojunction of said doping and of the composition.
The attached FIG. 4 shows on the one hand the variations of the N doping (D.sub.N) as a function of the depth Z (FIG. 3) of the semiconductor material and on the other hand the variations of the composition x of said material as a function of Z.
Although apparently very attractive, such a solution causes numerous production difficulties, because all the parameters have to be precisely adjusted. Thus, the following conditions have to be fulfilled:
the electric junction (passage from type N material to type P material) and the metallurgical junction (maximum variation of compositions x) must coincide;
the doping D.sub.N of zone N of composition x.sub.2 must be sufficiently high; and
the variation of composition x must not be too gradual compared with the variation of the doping D.sub.N.
It is merely necessary for one of these conditions not to be fulfilled for there to be a potential barrier which opposes the passage of electrons through the heterojunction. These electrons are produced as a result of an irradiation in the semiconductor of composition of x.sub.1 with a small forbidden band width Eg.sub.1.
Details of other known HgCdTe detectors using a continuous variation of composition x in accordance with the thickness of the semiconductor layer in the article by E. IGRAS and J. PIOTROWSKI "A new CdHgTe Photodiode Type with Protected Junction Surface" which was published in Optica Applicata VI, 3, pp.99-103, 1976. In these detectors, the maximum value of x occurs on the interface with the insulant. However, in the case of contacting on HgCdTe of type N with a large forbidden band width, the same sensitivity loss at high wavelengths persists.